Of the groups you mentioned, several invertebrate phyla primarily consist of species that inhabit saltwater environments. Here’s a brief overview:
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Cnidaria: This phylum includes organisms like jellyfish, sea anemones, and corals, most of which live in marine (saltwater) environments, although some species can also be found in freshwater.
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Sponges (Porifera): While most sponges are marine, there are some freshwater species as well. However, the majority of sponge diversity is found in saltwater.
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Mollusks: This is a diverse phylum that includes many marine species (like octopuses, squids, and clams), as well as freshwater and terrestrial species. Therefore, not all mollusks are exclusively marine.
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Echinoderms: This phylum includes starfish, sea urchins, and sea cucumbers, which are all exclusively marine organisms.
In summary, while Cnidaria, sponges, and echinoderms primarily consist of marine species, mollusks include both marine and non-marine species. Among those listed, the phyla that are predominantly marine are cnidarians, sponges, and echinoderms. The best answer to your question would be echinoderms, as they are exclusively marine.